This invention relates to an apparatus and method for sharpening disc blades, and, more particularly, disc blades of saws that transversely sever multi-ply material such as logs of bathroom tissue and kitchen towels and bolts of folded facial tissue and toweling.
Some of the first saws were the so-called "Gilbertville" saws, as described in U.S. Pat. Nos. 2,766,566 and 2,879,633. Those patents taught the use of air cylinder actuated grinder assemblies, with non-driven (idling) and driven grinding wheels, respectively. Although the air cylinder can be considered a spring, because of the force used to move and then load the grinding wheel against the blade and the need for a support on the opposite side of the blade, the grinding wheel was actually held rigidly against the blade.
As saw speeds increased to keep up with production increases, saw improvements were made as taught in co-owned U.S. Pat. Nos. 4,041,813, 4,173,846, 4,347,771, 4,584,917, 4,821,613, and 5,152,203. The '813, '846, 917, and '203 patents teach the use of air cylinder actuated grinder assemblies and loading the assembly against a fixed stop, thereby loading the grinding wheel onto the blade. The '813 patent also teaches driven grinding wheels, in place of idling wheels. The '771 patent teaches the use of light spring pressure, no stop, plus idling grinding wheels for blade sharpening and reduced blade scalloping. This was an improvement to the system as taught in the '813 patent. The '613 patent adds the teaching of blade speed oscillation to the teachings of '771 to reduce blade scalloping. It is noted that a blade is considered scalloped when the outside diameter is no longer a circle, but begins to look like a series of flats around the blade.
With the increased use of recycled substrates for the web and core board, plus wider webs and higher production speeds, saw demands have increased further. Present saws must deal with more impurities in the web, cut faster, and cut through more lanes of product with each pass. This increases the demand on the grinding wheels to keep the blade sharp, without causing or increasing unwanted blade scalloping. As this demand has increased, so has the need for the saw adjuster to set the relationship of the grinding wheels to the blade more consistently. The need for a more consistent grinding process, plus higher quality blades and grinding wheels has also grown.
A problem with idling grinding wheel assemblies is that the grinding process is not as controlled because each grinding wheel grinds at its own rate based on its rotational speed. The grinding wheel rotational speed is a function of the grinding wheel to blade overlap and pressure setting, the friction in the assembly, component manufacturing tolerances, component wear, and contamination as a result of the product cutting and blade sharpening processes.
When the grinding wheel assembly is loaded against a fixed stop, the blade conforms to the grinding wheel. As the blade flatness run out increases, the grinding force between the blade and grinding wheel changes as the blade rotates. Grinding wheel flatness runout can also add to this grinding force variation. These factors, along with the requirement of more aggressive grinding, can lead to blade scalloping, which can increase the grinding force variation even further. Blade scalloping can be compared to washboard on a dirt road, with the vehicle tires representing the grinding wheels. When the scalloping becomes pronounced enough, the blade must be replaced, as sharpening is no longer feasible.